1,320 research outputs found
Spectro-Perfectionism: An Algorithmic Framework for Photon Noise-Limited Extraction of Optical Fiber Spectroscopy
We describe a new algorithm for the "perfect" extraction of one-dimensional
spectra from two-dimensional (2D) digital images of optical fiber
spectrographs, based on accurate 2D forward modeling of the raw pixel data. The
algorithm is correct for arbitrarily complicated 2D point-spread functions
(PSFs), as compared to the traditional optimal extraction algorithm, which is
only correct for a limited class of separable PSFs. The algorithm results in
statistically independent extracted samples in the 1D spectrum, and preserves
the full native resolution of the 2D spectrograph without degradation. Both the
statistical errors and the 1D resolution of the extracted spectrum are
accurately determined, allowing a correct chi-squared comparison of any model
spectrum with the data. Using a model PSF similar to that found in the red
channel of the Sloan Digital Sky Survey spectrograph, we compare the
performance of our algorithm to that of cross-section based optimal extraction,
and also demonstrate that our method allows coaddition and foreground
estimation to be carried out as an integral part of the extraction step. This
work demonstrates the feasibility of current- and next-generation multi-fiber
spectrographs for faint galaxy surveys even in the presence of strong night-sky
foregrounds. We describe the handling of subtleties arising from fiber-to-fiber
crosstalk, discuss some of the likely challenges in deploying our method to the
analysis of a full-scale survey, and note that our algorithm could be
generalized into an optimal method for the rectification and combination of
astronomical imaging data.Comment: 9 pages, 4 figures, emulateapj; minor corrections and clarifications;
to be published in the PAS
Dry mergers and the formation of early-type galaxies: constraints from lensing and dynamics
Dissipationless (gas-free or "dry") mergers have been suggested to play a
major role in the formation and evolution of early-type galaxies, particularly
in growing their mass and size without altering their stellar populations. We
perform a new test of the dry merger hypothesis by comparing N-body simulations
of realistic systems to empirical constraints provided by recent studies of
lens early-type galaxies. We find that major and minor dry mergers: i) preserve
the nearly isothermal structure of early-type galaxies within the observed
scatter; ii) do not change more than the observed scatter the ratio between
total mass M and "virial" mass R_e*sigma/2G (where R_e is the half-light radius
and sigma the projected velocity dispersion); iii) increase strongly galaxy
sizes [as M^(0.85+/-0.17)] and weakly velocity dispersions [as M^(0.06+/-0.08)]
with mass, thus moving galaxies away from the local observed M-R_e and M-sigma
relations; iv) introduce substantial scatter in the M-R_e and M-sigma
relations. Our findings imply that, unless there is a high degree of fine
tuning of the mix of progenitors and types of interactions, present-day massive
early-type galaxies cannot have assembled more than ~50% of their mass, and
increased their size by more than a factor ~1.8, via dry merging.Comment: ApJ, accepted. 16 pages, 11 figure
Quantifying the Biases of Spectroscopically Selected Gravitational Lenses
Spectroscopic selection has been the most productive technique for the
selection of galaxy-scale strong gravitational lens systems with known
redshifts. Statistically significant samples of strong lenses provide a
powerful method for measuring the mass-density parameters of the lensing
population, but results can only be generalized to the parent population if the
lensing selection biases are sufficiently understood. We perform controlled
Monte Carlo simulations of spectroscopic lens surveys in order to quantify the
bias of lenses relative to parent galaxies in velocity dispersion, mass axis
ratio, and mass density profile. For parameters typical of the SLACS and BELLS
surveys, we find: (1) no significant mass axis ratio detection bias of lenses
relative to parent galaxies; (2) a very small detection bias toward shallow
mass density profiles, which is likely negligible compared to other sources of
uncertainty in this parameter; (3) a detection bias towards smaller Einstein
radius for systems drawn from parent populations with group- and cluster-scale
lensing masses; and (4) a lens-modeling bias towards larger velocity
dispersions for systems drawn from parent samples with sub-arcsecond mean
Einstein radii. This last finding indicates that the incorporation of
velocity-dispersion upper limits of \textit{non-lenses} is an important
ingredient for unbiased analyses of spectroscopically selected lens samples. In
general we find that the completeness of spectroscopic lens surveys in the
plane of Einstein radius and mass-density profile power-law index is quite
uniform, up to a sharp drop in the region of large Einstein radius and steep
mass density profile, and hence that such surveys are ideally suited to the
study of massive field galaxies.Comment: Accepted for publication in Astrophys. J., June 7, 2012. In press. 9
pages, 5 figures, 1 tabl
Prediction of Supernova Rates in Known Galaxy-galaxy Strong-lens Systems
We propose a new strategy of finding strongly-lensed supernovae (SNe) by
monitoring known galaxy-scale strong-lens systems. Strongly lensed SNe are
potentially powerful tools for the study of cosmology, galaxy evolution, and
stellar populations, but they are extremely rare. By targeting known strongly
lensed starforming galaxies, our strategy significantly boosts the detection
efficiency for lensed SNe compared to a blind search. As a reference sample, we
compile the 128 galaxy-galaxy strong-lens systems from the Sloan Lens ACS
Survey (SLACS), the SLACS for the Masses Survey, and the Baryon Oscillation
Spectroscopic Survey Emission-Line Lens Survey. Within this sample, we estimate
the rates of strongly-lensed Type Ia SN (SNIa) and core-collapse SN (CCSN) to
be and events per year, respectively. The lensed
SN images are expected to be widely separated with a median separation of 2
arcsec. Assuming a conservative fiducial lensing magnification factor of 5 for
the most highly magnified SN image, we forecast that a monitoring program with
a single-visit depth of 24.7 mag (5 point source, band) and a
cadence of 5 days can detect 0.49 strongly-lensed SNIa event and 2.1
strongly-lensed CCSN events per year within this sample. Our proposed
targeted-search strategy is particularly useful for prompt and efficient
identifications and follow-up observations of strongly-lensed SN candidates. It
also allows telescopes with small field of views and limited time to
efficiently discover strongly-lensed SNe with a pencil-beam scanning strategy.Comment: 14 pages, 5 figures, ApJ in pres
Galaxy-Scale Strong Lensing Tests of Gravity and Geometric Cosmology: Constraints and Systematic Limitations
Galaxy-scale strong gravitational lenses with measured stellar velocity
dispersions allow a test of the weak-field metric on kiloparsec scales and a
geometric measurement of the cosmological distance-redshift relation, provided
that the mass-dynamical structure of the lensing galaxies can be independently
constrained to a sufficient degree. We combine data on 53 galaxy-scale strong
lenses from the Sloan Lens ACS Survey with a well-motivated fiducial set of
lens-galaxy parameters to find (1) a constraint on the post-Newtonian parameter
gamma = 1.01 +/- 0.05 and (2) a determination of Omega_Lambda = 0.75 +/- 0.17
under the assumption of a flat universe. These constraints assume that the
underlying observations and priors are free of systematic error. We evaluate
the sensitivity of these results to systematic uncertainties in (1) total
mass-profile shape, (2) velocity anisotropy, (3) light-profile shape, and (4)
stellar velocity dispersion. Based on these sensitivities, we conclude that
while such strong-lens samples can in principle provide an important tool for
testing general relativity and cosmology, they are unlikely to yield precision
measurements of gamma and Omega_Lambda unless the properties of the lensing
galaxies are independently constrained with substantially greater accuracy than
at present.Comment: 8 pages, 5 figures; Accepted to Ap
The Sloan Lens ACS Survey. I. A Large Spectroscopically Selected Sample of Massive Early-Type Lens Galaxies
The Sloan Lens ACS (SLACS) Survey is an efficient Hubble Space Telescope
Snapshot imaging survey for new galaxy-scale strong gravitational lenses. The
targeted lens candidates are selected spectroscopically from within the Sloan
Digital Sky Survey (SDSS) database of galaxy spectra for having multiple
nebular emission lines at a redshift significantly higher than that of the SDSS
target galaxy. In this paper, we present a catalog of 19 newly discovered
gravitational lenses, along with 9 other observed candidate systems that are
either possible lenses, non-lenses, or non-detections. The survey efficiency is
thus >=68%. We also present Gemini and Magellan IFU data for 9 of the SLACS
targets, which further support the lensing interpretation. A new method for the
effective subtraction of foreground galaxy images to reveal faint background
features is presented. We show that the SLACS lens galaxies have colors and
ellipticities typical of the spectroscopic parent sample from which they are
drawn (SDSS luminous red galaxies and quiescent main-sample galaxies), but are
somewhat brighter and more centrally concentrated. Several explanations for the
latter bias are suggested. The SLACS survey provides the first statistically
significant and homogeneously selected sample of bright early-type lens
galaxies, furnishing a powerful probe of the structure of early-type galaxies
within the half-light radius. The high confirmation rate of lenses in the SLACS
survey suggests consideration of spectroscopic lens discovery as an explicit
science goal of future spectroscopic galaxy surveys (abridged).Comment: ApJ, in press. 20 pages, numerous figures, uses emulateapj. Replaced
to include full-resolution spectro figures. Version with full-resolution
imaging figures available at
http://www.cfa.harvard.edu/~abolton/slacs1_hires.pdf (PDF) or at
http://www.cfa.harvard.edu/~abolton/slacs1_hires.ps.gz (PS). Additional SLACS
survey info at http://www.slacs.or
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